Glow lamp stabilizing system



' w. w. MOE

GLow LAMP STABILIZING SYSTEM April 1, 195s Filed June 1, 1954 2 Sheets-Sheet 1 Filed lJune l, 1954 2 Sheets-Sheet 2 NVENTOR WILLIAM WEST MGI-:

-IOO

HIS ATTORNEYS United States Patent GLOW LAMP STABILIZING SYSTEM William West Moe, Stratford, Conn., assignor to lime, noorporated, New York, N. Y., a corporation of New ork Application June 1, 1954, Serial No. 433,435

14 Claims. (Cl. 315-169) The present invention relates generally to electrical systems in which electric signal variations are converted into light variations by glow lamps, and more particularly to improvements in such systems whereby a stabilized amplitude ratio is maintained between the electric signal variations and the light variations.

ln facsimile systems, such as the color facsimile systern dis-closed in the applicants copending application, Serial No. 251,898 (filed October 18, 1951) electric signals, having amplitude variations representing visual image information are applied to a glow tube used as a lamp whose light is directed upon a photosensitive medium. The intensity of the light emitted from-the glow lamp varies in accordance with the .amplitude of the applied signals with the result that there is exposed upon the medium the visual image information carried by the signals. It will be evident that for faithful reproduction of the visual image information on the medium, it is desirable and necessary to maintain a stabilized ratio between the amplitude of the signals applied to the glow lamp and the intensity of the light emitted therefrom.

As llong as the average level of the amplitude of the applied signals remains substantially steady, it has been found that the mentioned ratio remains at a stabilized value Within the tolerances required, When, however, there occurs a pronounced change in average signal level, it has been found that following this change the glow lamp undergoes what may be termed a time drift in the intensity of emitted light. If, for example, the average amplitude of the applied signal undergoes a sudden ncrease, the intensity of the emitted light undergoes a similar sudden increase, but thereafter, undergoes an additional creeping.increase over a time interval. ySimilarly, if the average amplitude of the applied signal Vundergoes a sudden decrease, the intensity of the emitted light undergoes a similar sudden decrease, but thereafter, undergoes an additional creeping decrease overa time interval, Thus, in both situations a change in average level of the applied signal is followed by a transition period during which a variance occurs in the ratio between e,

the amplitude of the signals applied to the glow lamp and the intensity of the light responsively emitted therefrom.

While it will be understood that the accuracy of applicants explanation of why the observed glow lamp drift occurs in no way affects the merits of the present invention, it is believed that thvedescribed drift is caused by a sensitivity of the glow lamp to thermal conditions. When the current through the glow lamp is increased, this increased current causes a heating of the glow lamp, which in turn increases the light emitting capability thereof relative to the amount of current passing therethrough. Conversely, when there is a decrease in the current pass-V ing through the glow lamp, the glow lamp is cooled to decrease the light emitting capability thereof relative to `the amount of current passing therethrough. .The hypothesis of a thermal origin for the light intensity drift explains the fact that, whether the current increases or decreases, if thereafter the current remains constant, the' 2,829,313 Patented Apr. 1v, 1958` ratio between this current andthe light emitted ulti'- mately reaches a steady state value, the reason being that ultimately the glow lamp reaches a new state of thermal equilibrium with its environment. The hypothesis of thermal origin also explains the fact that the mentioned light intensity drift becomes significant only when there is a change in average level of the applied signal. High frequency variations in the signal around the same average level do not result in drift for the reason that such high frequency variations cause no more than a negligible change in the thermal condition of the glow lamp.

The mentioned variance in the ratio between the current exciting a glow lamp and the light emitted therefrom is highly undesirable in facsimile reproduction for the reason that the ratio variance may cause a clearly noticeable spurious streaking of the reproduced Visual subject exposed upony the photosensitive medium. Not only is the streaking in itself undesirable, but in addition, in order to keep the streaking within the minimum bounds of acceptability for the reproduced visual image, it has been necessary in the past to operate glow lamps at considerably lower current values than those at which they would satisfactorily operate if the drift effect were not present (it being found with decreased ycurrent values that the drift effect diminishes by a factor which is greater than the decrease in current). Accordingly, as another disadvantage, the mentioned ratio variance precludes the use of the full capabilities of glow lamps.

lt is` an object of the invention, accordingly, to provide new and improved systems in which electric signals are converted into light by a glow lamp in a manner free of the above-noted deficiencies.

Another object of the invention is to provide systems of the above character wherein a stabilized ratio is maintained between the amplitude of the signals supplied as an input to the glow lamp and the intensity'of the light responsively emitted by the glow lamp.

A further object of the invention is to provide systems of the above character wherein the glow lamp may be excited over a greater light output range than that previously practical.

A still further object of the invention is to provide systems of the above character which are adaptable to diiferent type conditions encountered in the described converting action of the glow lamp.

' These and other objects of the invention are attained by providing, in association with an electric signal channel which supplies the visual image signals to the glow lamp, an electric circuit means which is responsive to these visual image signals to generate a correction signal which varies in accordance ywith the time vdrift in intensity of the light emitted by the glow lamp. In conjunction with this electric circuit means there is provided a feedback means responsive to the mentioned correction signal for varying the amplitude of the visual image signals in the channel by an amount which maintains stabilized the ratio between the amplitude of input signal to the glow lamp and the intensity of the light responsively emitted therefrom.

The invention may be better understood from the following detailed description of representative embodi* ments thereof, taken in conjunction with the accompanying drawings, in which: l I

Fig. l is a diagram partly in block and partly in sche-, matic of the color facsimile system disclosed in the aforementioned co-pending application, together with the Iadditional system which is the subject of the present invention; Y

Fig. 2 is a set of graphs of aid in explaining the significance of the present invention; and

Fig. 3 is a schematic diagram showing an embodiembodiment shown in Fig. 1. ,f

It will be understood in the description to follow that similar elements are designated by similar numerals, prime and double prime suffixes being used for the purpose of distinguishing the elements.

Referring now to Fig. l, which shows in simple form the color facsimile system disclosed in much greater detail in the applicants aforementioned co-pending U. S. application, Serial No. 251,898, the numeral 1,0 ldesignates a color analyzing scanner which, in a well-known manner, is adapted to scan a colored original visual subject (not shown), and to convert the color information presented by the visual subject intorthree electricvisual image signals representing three `primary component colors, yellow, magenta and cyan. image signals are respectively supplied via the leads 11, 11', `11", to three electric signal channels 12, 12', 12, hereafter referred to as the yellow, magenta and cyan signal channels. through their corresponding channels are modified in various respects (hereafter described in more detail). Ultimately the Vmodified colored signals emerge from the channels on Aoutput leads 13, 13', 13, to respectively drive the yellow, vmagenta `and cyan glow lamps 14, 14',

14" in the sense that the intensity of the light emitted by each glow lamp isa function of the amplitude of the corresponding color signal. The light emitted from the glow lamps 14, 1,4', 14" is directed in a well-known manner onto three corresponding photographic emulsions (not shown) to impress thereon the visual image information carried by the respective color signals. The color facsimile system of Fig. 1 is thus adapted to produce three color component reproductions of the original colored visual subject presented to scanner 10.

In Fig. 1, the yellow channel 12 is, with regard to the present invention, essentially similar to the .magenta channel 12' and the cyan channel 12". Accordingly,

The three color signals in passing ri'hese three visual t' unless otherwise noted, `the following description of yellow channel 12 applies as properly to the other electric signal channels. Y

To enlarge upon the details of the system of Fig. 1, the yellow visual information .from scanner 10 appears upon lead 11 in the form of amplitude variations of a direct current signal. signal ,is supplied by lead 11 as an input to the front end section (shown in block form in Fig. 1) of yellow channel 12. Section .20(as well as modifying the signal in ways not of present'interest) converts the visual information of the yellow signal into the form of modulations ron a high frequency (150 kc.) carrier, all as described in the aforementioned zo-pending application. This modulated high frequency carrier signal is fed from the output of section 20 via lead 21`to a successive channel section 22 shown in Fig. l within the dotted lines 23, 24, 2S, 26. n

Section 22 represents, as hereafter more fully described, a section ofthe yellow signal channel 12 which has a controllable signal transfer ratio. In section 22 the kyellow signal in the ,modulated high frequency carrier form is applied as an input to a pentodel) by means of a coupling capacitor 31 which impresses the signal upon the control grid 32 of the pentode. The plate33 of pentode is coupled through resistorsv 34, 3S to a source of ,positive voltage supply (notshown) and `is also coupled through a capacitor 36 with a parellel resonant circuit `37 tuned to the carrier `frequency and coupled in effect at this frequency between the plate 33and ground to form .a plate load. i Pentode 30 is-otherwise coupled to act as an amplifier tube. Accordingly, ,pentode 30 and parallel resonant circuit 37 together form a tuned amplifier stage, the output of which appears across res- This yellow visual image 4 surately with the effective impedance at the carrier frequency of the plate load. t

The output from the tuned amplifier stage is fed via a coupling capacitor 38 and lead 39 to a succeeding scction 4t) in the yellow signal channel.

Section 40 comprises, in the order of signal flow direction, an amplifier stage and a cathode follower stage (neither being shown in detail), both of which stages are conventional and perform their usual and well-known operations. From section 4) the yellow signal in modulated high frequency carrier form is advanced to a rectifier and voltage doubler section 41 which acts as a demodulator to convert the yellow visual image information back into the `form of amplitude variations of a direct current signal upon the output lead42 for section 41;

The direct current yellow 'signal upon lead 42 is applied to the control electrode 43 of an electron discharge device 44 (as, say, a pentode kof the 6AC7 type) within a channel section 4S outlined `by the dotted lines 46, 47, 48, 49. The cathode 50.of vacuum tube 44 is coupled to ground through a variable resistor 51 within another channel section 52 outlined `by the dotted lines 48', 53, 54, S5. Thus, the visual image information of the signal upon lead 42 will appear within channel section 52 in the form of a voltage across the variable resistor 51. The plate of vacuum tube 44 is in circuit with a positive voltage supply (not shown) through a serial coupling withfthe yellow glow lamp 14 which may be a neon crater lamp of the R1l35 type manufactured by the Sylvania Electric Co. Since glow lamp 14 and the anodecathode path of tube 44 are thus in serial coupling, it will be seen that variation of the signal on lead 43 not only produces, in a well-known manner, a commensurate variation of ,the anode-cathode current through tube 44, buty Valso producesl a corresponding variation in current through `glo'w lamp 14. This current variation through glow lamp 14 `causes a related change 'in the intensity of the light emitted by the glow lamp. Accordingly, the light output of the glow lamp varies as a fun-ction of the input signal to tube 44, or, equivalently, as a function of the voltage developed across resistor 51 within channel section 52.

It will be understood that various circuits disclosed in the aforementioned co-pending application, but not pertinent to an understanding of the present invention, have been omitted from the showing and description of Fig. l.

The foregoing description pertains to the color facsimile system as disclosed in the aforementioned copending application. Coming now to a consideration of the `present invention, with the heretofore described facsimile system, in the presence of a pronounced change in average level of the yellow signal, the glow lamp 14 undergoes, as described, a time drift in intensity of the light emitted therefrom. For a 'better understanding of the nature of this time drift in light intensity, reference is made to Fig. 2, wherein the horizontal coordinate in the yseveral shown graphs represents elapsed time, and the vertical coordinate represents signal level.

Considering first graph A of this figure, the solid line 60V represents the amount of current passing through glow lamp 14, whereas the dotted line 61 represents the inten- Y sity of the light responsively emitted therefrom, the values of currentamplitude and Ilight intensity both being exonant circuit 37. As is well known, the amplification t ing to the same numerical scale along the vertical coordinate of graph A. It will be appreciated that the current andlight intensity representations are idealized for clearer explanation to show simple assumed levels for/these quantities. 'In practice, the current and light intensitywould be much more irregular and complexthan the showings thereof in graph A. However, the same principles would still apply.

`'Considering the significance of .graph A, at time to both the glow lamp current I0 and the glow lamp light `intensity L0` are assumed. to be at zero value corresponding to minimum brightness or Vvblack -for the visual image information. Assume now that at time t1 the visual image brightness becomes maximum or white Under this condition, the glow lamp current I increases suddenly to a value I1, while the light intensity L correspondingly increases to a value L1, the units of measure lbeing selected so that I1 equals L1 in graph A. From time t1 to a later time t'1, the glow lamp current I remains constant at its initial increased value I1. During this same transitional time interval, however, the light intensity L instead of remaining constant at its initially increased value L1, continues to drift upward with a gradual leveling oif until it reaches the value Ll at t1, the time at which the drift in light intensity can be considered to become negligible. Following` time t1, both the current I and the light intensity L remain at their values I1 and Ll, so that the ratio L/I becomes stabilized at a value for the units of measure employed of 2O units (for L) `divided by 16 units (for I), or 1.25.

Assume now at time t2 that the glow lamp current I suddenly decreases from its value I1 of 16 units to a new value I2 of 4 units, there being a net decrease of 12 units. The light intensity L'1 undergoes a responsive decrease of 12 units from its value L1 of 20 units to a new value L2 of 8 units. From time t2 to a later time t2, the current I remains constant at its initially decreased value I2. During this same transitionaltime interval, however, the light intensity L drifts downward from its initially decreased value L2 with gradual leveling olf until at time t'z it reaches the value L'z, the value at which the drift can be considered to be negligible. Following time t2, the current I and light intensi-ty L stay constant at their values I2 and L'2 with the result that the ratio L/I is re-stabilized at the value of 5 units (for L) dividedby 4 units (for I), or 1.25.

Note that where there is a current decrease, as at time t2, it takes a longer interval (f2-t2) for ratio stabilization to be attained than the interval (fr-tl) required where there is a current increase. This longer transitional time period for a current decrease can be explained according to the thermal origin theory of the light intensity drift by the fact that it takes a glow lamp a longer period to cool in response to decreased current than to heat in response to increased current.

From the foregoing it will be seen that with the facsimile system as heretofore described, after a lapse of time following a pronounced change in average level of the glow lamp current, the current I and the intensity L of the light responsively emitted willbe quantitatively related according to a ratio having a stabilized value.

It will also be seen, however, that during transitionall time intervals following the level change, the ratio between output light and input current, instead of being stabilized, will undergo a considerable variance. Note that this ratio variance eiect is particularly aggravated when, as at time t2, the glow lamp has been in operation for awhile, the ratio L/I at this time being 8 units (for L) over 4 units (for I), or 2.00 in contrast to its later assumed value of 1.25.

The -ratio variance is, of course, caused by the light intensity drift which over these transitional time intervals changes curvilinearly, and with continuously decreasing absolute slope from its initial value to its limiting or leveled-oir value. To a reasonable approximation it has been found that the light intensity transition curve can be expressed by the exponential function:

Where the quantities Lc, k1, Ic, e, p1 and lrepresent, respectively, the instantaneous value of the change in light intensity, a multiplying constant, the change in average current level which initiates the drift, the Napierian base 2.71828, an exponential constant, and elapsed time. It will be understood that the value Ic is given plus and minus arithmetic signs when it represents7 respectively, an increase and 'a decrease, and that the exponential constant' p1 has different values m1 and n1 for an increase and a decrease. If the constant k1 is given a value 0.25, the expression set forth above is appropriate to the transitional changes in light intensity shown in graph A.

To eliminate the variance in the ratio L/I, there is developed according to the present invention a correction voltage which is shown by line 63 in graph B. This correction voltage bears an analog relation to the described light intensity dri-ft in that the correction voltage simulates the light intensity transition curve. Thus, the correction vvoltage also changes curvilinearly with decreasing absolute slope over the time intervals following a pronounced change in average level of the visual image signal. More specifically, if, as shown in graph B, the amplitude ofl correction voltage is denoted by E and the change in amplitude during a transitional time interval by Ec, the instantaneous value of Ec between steady state values Es of E preceding and following an average level change of the visual image signal, can be represented by the expression:

Ec=k2Vc(1-e-P2t) wherein e and t represent the same quantities as before, k2 and p2 represent constants and Vc represents the change in average level of the visual image signal when this average level change is expressed in voltage form. Note that the quantity Vc has plus and minus arithmetic signs when, respectively, the average signal level increases and decreases.

The correction voltage so developed is utilized, as shown by line 64 in graph C, to cause a magnitude change Mc of the visual image signals in a sense opposing the direction of change of the average signal level. When the visual image signals are so varied, the magnitude change induced in the signals by the correction voltage offsets the drift in glow lamp light intensity induced by the change in average signal level for the reason that the changed signal strength decreases (or increases) the emitted light by an amount approximately equal to the increased (or decreased) extraneous increment of light caused by the drift. Accordingly, by using the described correction voltage with proper values for the constants k2 and p2 and a proper quantitative relation between correction voltage values and resulting amplitude values of the visual signal, the transition curve of emitted light intensity L is substantially eliminated so that the ratio L/l remains stabilized within the usual tolerances required, following average level changes of the visual signals.

As stated, the foregoing graphical representations of light intensity and cur-rent amplitude have been idealized for greater convenience of explanation. One of the simplifying assumptions which has been made is that the described channel will have a standardized operating characteristic of light intensity vs. signal amplitude which is both of a linear nature and of the same L/l value for any point on the characteristic. In other words, if,` in the situation illustrated by graph A where no correction voltage is applied, the light intensity L is plotted against the signal amplitude I as the latter is changed slowly enough to eliminate the time drift factor of the lamp, the operating characteristic thereby obtained will be a straight line which is formed of (L,I) points, and which passes through the origin of the plot such that the ratio L/l has the same value 1for all points on the line as, say, at the respective points where I equals 16 units and 4 units.v

Also, if in the situation represented by graph C where correction voltage is applied, the operating characteristic of light intensity vs. signal amplitude is obtained in like manner (but including the effect of the correction voltage), the result will be a characteristic which again takes the form of a straight line passing through the origin of the plot. The characteristic obtained when correction voltage is applied is, of course,'the characteristic of Principal interest to the present invention. t

While it maybe advantageous for the last-named characteristic to be linear and to pass through the origin of the plot, the present invention is not concerned so much with these features as with the feature that (irrespective of its linearity or the values obtained for the L/I ratio from point to point thereon) the characteristic of light intensity vs. signal amplitude shall be one which remains relatively invariable or standardized despite occurrences of the described time drift in intensity of the lamp. To state it another way, it is the purpose of the present invention to assure that, whatever may be the prole of the intensity-amplitude characteristic as plotted from` (LJ) point to (L,l) point, the characteristic shall, at all times, be defined by approximately the same set of (Ll points even though a time drift of intensity occurs in the lamp. This is whatis meant lby the statement made above that the transition curve of emitted light intensity L is substantially eliminated so' that the ratio L/r' ren mains stabilizer The ratio L/I is considered to be stabilized when, for any given (LJ) point on the intensityamplitude characteristic, the value of the ratio of quantities L and. l, which are represented by this point, is a value which remains substantially constant despite the occurrence of a drift in the intensity of the lamp.l

The results obtained by the described generation and feeding back of the correction voltage are shown in graph D by the line 65 representing light intensity. Note that in graph D, while the transition curve for L is completely eliminated between times t1 and ll, a residual transition curve remains between the times t2 and t2. This residual` v transition curve may be attributed to the fact that in the correction Voltage of graph B the same value for the exponential constant p2 is used for both a correction voltage increase and decrease, whereas, as described, ,the light intensity drift curves are characterized by dilerent values m1 and filter their exponential constant p1 when the visual signal average level undergoes, respectively, an increase and a decrease.

To develop the correction voltage shown in graph B for varying the visual signal amplitude as shown in `graph C, there is provided, in accordance with the invention, a voltage modifying circuit 7G (Fig. l) for theyellow signal channel 12. In Voltage modifying circuit 79, a high impedance resistor 71 and a capacitor 72 of large capacitance value are coupled together to form an integrating means or integrating circuit, it being understood that the integrating circuitacts as an exponential voltage generator, rather than performing exact integration in a mathematical sense. For proper action as an integrating circuit, resistor 7l may have a value of one megohrn, while capacitor 72 may have a value of 16 microfarads.

The two mentioned electrical elements form a serial coupling which, by a ground connection at one end and by a lead 73 at the other, is coupled to receive the voltage developed in channel section S2 by being connected across cathode resistor 51 of pentode 44. It will be recalled that this cathode resistor voltage is a manifestation ot' the visual image signal and will accurately reflect changes in level thereof. Accordingly, a pronounced increase in the average signal level will cause capacitor 72 to charge with corresponding `development of a rising exponential voltage across its terminals, while a sudden decrease in average signal level causes capacitor 72 to on a high frequency carrier.

the signal transfer ratio of Section 22 of yellow channel 1 2. It will be recalled that in channel section ZZ the yellow visual image signal is in the form` of modulations Thyrite resistor 33 at the carrier frequency vis in effect coupled across the resonant circuit 37 `by a connecting lead84 at one end` of the resistor, and by a radio frequency bypass capacitor S5 connected between the other end of the resistor and ground. Thyrite resistor 83, through lead 8d, also is in path tor current developed in Voltage modifying .r t Vresponsive to the correction voltage generated across capacitor 72, the mentioned current owing from junction Et) through thyrite resistors 8l, 82, thyrite resistor 8 3 upwards through lead 74, downwards through resonant circuit 37 and returning to ground.

las is well known, a thyrite resistor passes current as an exponential function of the applied Voltage. Accordingly, a thyrite resistor may be said to have a non-linear resistance which varies in an opposite sense to the variance in the current passing therethrough. Assuming that an increasing corrctionvoltage appears across capacitor 72, this voltage causes a current which increases, roughly, in an exponential manner to flow through resistor 83 with the result that the resistance thereof is correspondingly decreased. The resistor 83 in terms of the modulated carrier color signal in channel section 22 forms (in conjunction with resonant circuit 37) a significant component of the plate load for amplier pentode discharge with corresponding development of a de creas` i' feedback circuit including the attenuating thyrite 4resistors i 81, -82 to another thyrite resistor 83 which acts to control Y 30. As described heretofore, the amplification factor of a pentode amplier stage varies in accordancefwith the impedance of the plate load. Accordingly, as the resistance of thyrite resistor 83 is decreased bythe current induced by the correctiontvoltage, the impedance at the carrier frequency of the plate load for pentode 30 is correspondingly decreased. it follows that the signal transfer ratio for the modulated carrier between theinput and output of channel section 22 will be varied to decrease in amplitude `the output signal relative to the input signal. Thus, it is seen that thyrite resistor 33 represents a means `for controlling the signal ytransfer ratio of the section,

if the signal transfer ratio of channel section 22 is so decreased, it is evident that the strength of the visual image signal appearing in direct` current form upon control grid 43 of pentode 44 will also be decreased. This decrease in signal strength at pentode 44 reduces the current through glow ylamp 14 to accordingly counteract (as shown in graph C, Fig. 2) the tendency of the glow lamp to dritt upward in emitted light intensity responsive to an increase in average level of the visual image signal.

in a manner analogous to that given by the description above, in the presence of a decreasing correction voltage appearing across capacitor 72, the effective rcsistance of thyrite resistor 83 is increased to increase the plate load impedance of pentode 30 in channel section 22 to in turn increase the signal transfer ratio of this section. Hence, the signal strength at pentode 44 will rise to increase the glow lamp current by an amount which counteracts the tendency of the glow lamp to have a falling drift in emitted light intensity after a sudden decrease in average level of the visual' signal.

T hyrite resistors S1, S2, which have the same voltagecurrent characteristics as thyrite resistor 83, represent circuit elements forattenuating the amount of correction voltage applied across resistor 83. Since all three resistors are of similar characteristic, it will be seen that the voltage across resistor S3 is directly proportional to the correction voltage even though the last-namedA resistor is non-linear and is in a series of voltage dividing resistors. It is not, however, essential to the present invention that strict proportionality be maintained between the correction voltage and the fraction thereof appearing across resistor 83. Y

The propervalues for the constants k2 and p3 in the above set forth expression for correction voltage, and

the proper quantitative relation'between Vc .(the voltage across cathode resistor 51) and Mc (the responsive change in magnitude of the visual image signal) may be established in a manner well known to theV art. To vary this quantitative relation, however, there is provided anadjustable control means in the form of a variable resistor 90 connected between the junction of thyrte resistors 82, 83 and ground. Adjustment of variable resistor 90 changes in effect the value of the constant k2 in the above set forth expression for the correction voltage.

Referring now to Fig. 3, there is shown an alternative embodiment of the invention associated with the yellow signal channel 12 (Fig. l), the same numerals being used to designate the elements in Figs. l and 3 which are the same. This alternative embodiment takes the form of a voltage modifying circuit 100 which, as before, receives over lead 73 an input signal from the cathode 50 of pentodeV 44, but which feeds back over return lead 84 its output signal to the yellow signal channel by applying its output signal through suppressor grid 101 of pentode 44 in channel section 45. In circuit organization, the voltage modifying circuit 100 includes a capacitor 102, a first resistor .103 and a second resistor 104 serially coupled in the order named between lead 73 and ground. These elements may have values of 16 microfarads, l megohm, and 3 megohms, respectively. The modifying circuit also includes a rectifier 105 in the form of a diode connected between the junction of resistors 103, 104 and ground'. The output lead 84 for the modifying circuit is Aconnected to the junction of capacitor 102 and resistor 103 to convey the signalat this point to the suppressor grid 101 of pentode 44. Note that (in contrast to Fig. l) in Fig. 3 the suppressor grid 101 is disconnected from the cathode 50 of the pentode.

With regard to operation of voltage modifying circuit 100, the cathode resistor S1 in section 52 of yellow signal channel 12 manifests the yellow visual image signal in the form of a voltage. Responsive to a sudden increase in the average level of this voltage, the suppressor grid 101, by virtue of its coupling through lead 84, capacitor 102 and lead 73 to cathode 50, will undergo a sudden corresponding increase in voltage for the reason that the potential between the terminals of capacitor 102 cannot change instantaneously. The transconductancc of pentode 44 varies in the same sense as the voltage on its suppressor grid. Hence, initially following a sudden increase in average signal level, pentode 44 will have a high transconductance permitting relatively high current ilow through its anode-cathode path, and correspondingly through the glow lamp 14. t

Following the sudden change in average level, however,` capacitor 102 begins to charge negatively by ow of current from the capacitor through resistor 103 and diode 105 to ground, the capacitor 102 and resistor 103 formtogether an integrating circuit which generates across the i capacitor a decreasing exponential voltage, shown by the line 66 for the interval (f1-t1) in graph E (Fig. 2). This decreasing exponential `or correction voltage is imparted by lead 84 to suppressor grid 101 to correspondingly decrease the transconductance of pentode 44. Thus, the current through glow lamp 14 correspondingly decreases to offset the tendency of the glow lamp to drift upward in emitted light intensity responsive to the average level increase (of the visual image signal fed to control grid 43 of pentode 44 by lead 42). As a result, there is effected, as shown by the line 67 (representing light intensity) in graph F (Fig. 2), an elimination of the transition curve which characterizes (as shown in graph A) the yellow signal channel in the absence of a voltage modifying circuit of the sort described.

In accordance with the foregoing description, it will be noted that, in the case `of the Fig. 2 embodiment, it is the channel section 45 (containing the pentode 44) which has the controllable signal transfer ratio. The ratio between the strength of the input signals on control electrode 43 and the strength of the'outpu't signals v(inv the form of anode-cathode current variations) from pentode 44 is controlled by the voltgae on suppressor grid 101 which governs the transconductance of the pentode.

It will be recalled that the Fig. 1 embodiment of the present invention -in the presence of a sudden decrease in averagesignal level results in a greatly diminished, but residually remaining light intensitydrift, shown in the interval between times t2 and t'2 for graph D. As heretofore stated, this residual drift is explainable in terms of the mathematical expression set forthabove for the transitional change in light intensity in that the exponential constant pl-has diiferent values m1 and nl for, respectively, an increase and decrease in average signal level. On the other hand, in the mathematical expression (set forth above) for the correction voltage, developed by the Fig. 1 embodiment, the exponential rconstant p2 has the same value, say, m2, for both an increase and a decrease in average signal level. While the residual drift shown in graph Dis so small as to be an insignificant factor in many facsimile system operations, there are other operations when the residual drift should be eliminated.

Such elimination is accomplished by the embodiment of Fig. 3.

Regarding the mode of counteracting residual drift, as stated, in the presence of an average signal level increase, the capacitor 102 in Fig. 3 charges negatively by a ow of current from the capacitor through resistor 103 and diode to ground. In this situation, diode 105 acts as a short-circuiting element between resistor 103 and ground, with the result that the exponential constant p2 for the correction voltage across capacitor 102 has a value m2 which is determined solely by the capacity and resistance of, respectively, capacitor 102 and resistor- 103, the resistance value of resistor 104 being entirely excluded from the determination. When, on the other hand, there occurs a ldecrease in average signal level, the capacitor 102 will commence to charge positively'by ow of current from ground through resistor 104, 1,03 and to the capacitor. In this latter situation, the diode 105 acts as an open circuit. Thus, in this latter situation, the exponential constant p2 for the correction voltage across capacitor 102 will have a value n2 determined by the resistance of resistor 104, as well as by the resistance and capacity of, respectively, resistor 103 and capacitor 102, the value n2 being greater than the value m2. Accordingly, the diode 105 represents a form of polarity discriminator means which imparts dilerent instantaneous valuesV to the slope of the transient correction voltage according to whether the average level of the visual image signal undergoes an increase or a decrease.

By the action of diode 105 and as shown by comparison of the intervals (f2-4Z) and (f1-t1) in graph E (Fig. 2), a decreasing correction voltage takes a longer timey to reach its limiting change in amplitude (at time t'2) than does an increasing correction voltage to reach its limiting change in amplitude (at time tl). yBy properly proportioning the circuit values for resistors 103, 104 and capacitor 102, respective curves for increasing' and decreasing correction voltages may be obtained, so that, as shown in graph F, the ratio L/I is substantially entirely stabilized for both an increase and also a decrease* in the average level ofthe visual image signal.

With the ratio L/I completely stabilized, as described, the Y aforementioned undesired streaking of the reproduced visual image can be fully eliminated. Also, the glow lamps can be operated over a much wider current range than that hitherto practical.

While the foregoing description has been addressed pri-V marily to ratio stabilizing circuits for onev particular (the yellow) electric signal channel of a facsimile svstem, it will'be understood that the same means may be employed to stabilize the glow lamp ratios of a plurality of color component channels. It will also bek understood that the present invention is of application in black-and-white'l 1'1 fascimile systems, and, in factin any system where a highly stable ratio isdesirable between .thelstrength of. a visual image signal applied toarglow lampfand theintensity of the light responsively emitted therefrom. Note also that the invention is ofapplication whether the correction` signal is applied to the visual imagosignal in rthe form of a modulated` high frequency carriery (theFig. l embodiment) `or in the formv of a varying direct current signal (the Fig. 3 embodiment). v

The embodiments shown'in the drawings andldescrib'ed herein are obviously susceptible `of considerable modification in form and detail within the spirit of the invention. The embodiments, therefore, are to be'regarded as illustrative only and not as limiting the vscope ofzthe following claims. 4

I claim:

1. A system for stabilizing the ratio between the amplitude of` visual image signals applied through an electric signal channel to develop `current in azgaseous discharge lamp andthe intensity .of light which is `responsively emitted from said lamp, and which may/transiently dri-fit in intensity followingza change in average level of said signals, said system comprising, electric circuit means responsive to change in average level of said visual image signals in said channel for'generating a transient correction signal in the form lof an electrical transient which is representative in instantaneous amplitude of the transient ,drift in intensity of said emitted light, and feedback means re-` sponsive to said correction signal for varying the amplitude of said visual image signals in said channel by an amount which maintains said ratio at a constant value following changes in average level of saidvisual image signals.

2. A system for stabilizing the ratio between the amplitude of visual image signals applied through an electric signal channel to develop current in a gaseous discharge lamp and the intensity of light which is responsively emitted from said lamp, and which may transiently drift in intensity following a change in average level of said signals, said system comprising, a iirst section of said channel adapted to manifest said visual image signals as ya voltage, electric circuit means responsive to a changey in average level of the visual imagesignal voltage in said first channel section for generating a transient correction voltage `which is representative in instantaneous amplitude of the transient drift in intensity of saidy emitted light, a second section in said channel having a controllable signal transfer ratio for visual image signals passing therethrough, and feedback means responsive to said correction voltage for controlling the signal transfer` ratio of said second channel section to maintain said ratio at a constant value following said change in level.

3. A system for stabilizing the ratio between the amplitude of visual image signals applied through an electric signal channel to develop current into a gaseous discharge lamp and the intensity of light which is responsively emitted from said lamp, and which may transiently drift in intensity following a change in average level of said signals, said system comprising, a iirstsection of said channel adapted to manifest said .visual image signals as a voltage, electric circuit means responsive to a change in average level of the visual image signal voltage in said first channel section for generating a curvilinearly changing transient correction voltage which in ampliture progresses with decreasing absolute slope towards a limiting change in amplitude commensurate with said change in level, and which is representative in instantaneous amplitude of the transient drift in intensity of said emitted light, a second section in said channel having Va controllable signal transfer ratio for visual image signals passing therethrough, and feedback means responsive to said correction voltagefor Icontrolling the signal Itransfer ratio of said second channel section to maintain said ratio at aconstant value following said change in level.

4. A system for stabilizing the ratio between the amplitude of visual image signals applied through an electric signal channel to developicurrent in a gaseous discharge lamp and the intensity of light which -is responsively emitted from said lamp, and which may transiently drift in intensity following a change in average level of said signals, said system comprising, a iirst section of said channel adapted to manifest said visual image signals as a voltage, electric circuit mean responsive to a change in average level ofthe visual image signal voltage in said rst channel section for generating a curvilinearly changing transient correction voltage which in amplitude progresses with decreasing absolute slope towards a limiting change in amplitude commensurate with said change in level, and which is representative in instantaneous amplitude of the transient drift in intensity of `said emitted light, polarity discriminator means selectively responsive to average level changes of diiferent sign for imparting different instantaneous slope values to saidcurvilinearly changing correction voltage, a second section of said channel having a controllable signal transfer ratio for visual image signals passing therethrough, and feedback means responsive to said correction voltage for controlling the signal transfer ratio of said second channel section to maintain said ratio at a constant value followingsaid change in level.

5. A system for stabilizing the ratio between the amplitude of visual image signals applied through an electric signal channel to develop current in a gaseous discharge lamp and the intensity of light which is responsively emitted as an output from said lamp, and which may transiently drift in intensity following a change in average level of said signals, said system comprising, a first section of said channel adapted to manifest said visual image signals as a voltage, electric circuit means responsive to a change in average level of the visual image signal voltage in said rst channel section for generating a curvilinearly changing transient correction voltage which in amplitude progresses with decreasing absolute slope towards a steady state amplitude commensurate with the signal level following said change, and which is representative `in instantaneous amplitude of the transient drift in intensity of said emitted light, a second section of said channel having, a controllable signal transfer ratio for visual image sig-4 nals passing therethrough, and feedback means responsive to said correction voltage for controlling the signal transfer ratioof Said second channel section to maintain said ratio at a constant value following said change in level. y

6. A system for stabilizing the ratio between the ainplitude of visual image signals applied through an electric signal channel to develop current in a gaseous discharge lamp and the intensity of the light responsivelyemittetl from said lamp, said system comprising, a iirst section of said channel adapted to manifest said visual image signals as a voltage, electric circuit means responsive to a change V in average level of the visual image signal voltage in said tirst channel section for generating atransient correction voltage E in accordance with the expression:

sewn-rrr) a controllable signal transfer ratio vfor visual image nals passing therethrough, andfecdback means responsive to said correction voltage for correspondingly varying the signal `.transfer ratio of saidsecond channel section. l

'47. A system as in claim 6 further characterized by adjustable control means coupled in circuit with said cor-` rection voltage generating means'to modify the voltage 13 developed thereby in a mode corresponding to assignment of diterent specific values to said constant k.

8. A system as in claim 6 further characterized by polarity discrirninator means coupled in circuit with said correction voltage generating means to modify the voltage developed thereby in a mode corresponding to assignment of different specific values to said constant p for different polarities of said average level change.

9. A system for stabilizing the ratio between visual image signals applied through an electric signal channel to develop current in a gaseous discharge lamp and the intensity of the light responsively emitted from said lamp, said system comprising, a iirst section of said channel having terminals and adapted to manifest said visual image signals as a voltage between said terminals of said section, a second section in said channel having an input and an output and having a controllable signal transfer ratio between said input and output for visual image voltage signals passing therethrough, and a voltage modifying circuit coupled at one end with said terminals and at the other end with said second channel section to control the signal transfer ratio thereof in response to the changes occurring in the average level of the voltage between said terminals to thereby modify the voltage of said signals at the output of said second channel section relative to the voltage of said signals at the input thereof, said response of said signal transfer ratio of said second channel section being a response which is in proportion to the amplitude of said changes in average level, and which is exponential with respect to time.

10. A system for stabilizing the ratio between visual image signals applied through an electric signal channel to develop current in a gaseous discharge lamp and the intensity of the light responsively emitted from said lamp, said system comprising, a rst section of said channel having terminals and adapted to manifest said visual image signals as a voltage between said terminals of said section, a first resistor and a capacitor serially coupled between said terminals as an average level integrating circuit, a second section in said channel having a controllable signal transfer ratio for visual image signals passing therethrough, and a feedback circuit coupled at one end across said capacitor and at the other end with said second channel section to control the signal transfer ratio thereof by the voltage across said capacitor.

ll. A system as in claim l further characterized by an adjustable amplitude control device for varying in accordance with its setting the relative amplitude of the voltage supplied by said feedback circuit to said second channel section.

l2. A system for stabilizing the ratio between visual image signals applied through an electric signal channel to develop current in a gaseous discharge lamp and the intensity of the light responsively emitted from said lamp, said system comprising, a first section of said channel having terminals and adapted to manifest said visual image signals as a voltage between said terminals of said section, a first resistor and a capacitor serially coupled between said terminals as an average level integrating circuit, a second resistor non-linear in resistance with applied voltage connected in said channel as a portion of a second section thereof and adapted to receive said visual image signals across its terminals, and a feedback circuit coupled at one end across said capacitor and at the other end across the terminals of said second resistor to control the resistance thereof by the voltage across said capacitor.

13. A system for stabilizing the ratio between visual image signals applied through an electric signal channel to develop current in a gaseous discharge lamp and the intensity of the light responsively emitted from said lamp, said system comprising, a first section of said channel having terminals and adapted to manifest said visual image signals as a voltage between said terminals of said section, a first resistor and a capacitor serially coupled between said terminals as an average level integrating circuit, an electron current control device connected in said channel as a second section thereof and having at least two control electrodes of which one electrode receives said visual image signals to correspondingly vary the current through said device, and a feedback circuit coupled at one end across said capacitor and at the other end with said other control electrode to control the transconductance of said device by the voltage across said capactor.

14. A system for stabilizing the ratio between visual image signals applied through an electric signal channel to develop current in a gaseous discharge lamp and the intensity of the light responsively emitted from said lamp, said system comprising, a first section of said channel having terminals and adapted to manifest said visual image signals as a voltage between said terminals of said section, a capacitor and two resistors serially coupled between said terminals as an average level integrating circuit, a rectifier in shunt with one of said resistors, a second section of said channel having a controllable signal transfer ratio for visual image signals passing therethrough, and a feedback circuit coupled at one end across said capacitor and at the other end with said second channel section to control the signal transfer ratio thereof by the voltage across said capacitor.

References Cited in the le of this patent UNITED STATES PATENTS Stark Nov. 27, 1945 Hough May 22, 1951 OTHER REFERENCES 

